This invention relates to a method for treating sintered alloy to form a protective layer on its surface. This method is especially useful for sintered alloy bodies having protrusions and depressions along their surfaces, and for sintered alloy bodies having complex structures and/or thin walls, such as a honeycomb structure.
To enhance corrosion resistance and lubrication ability, parts made of iron have undergone a water vapor treatment in which they are hold in pressurized steam at a temperature between about 500.degree. C. and about 600.degree. C. to form a coating of Fe.sub.3 O.sub.4 on its surfaces. However, this coating does not function as a protective layer against oxidation in higher temperatures.
Methods for forming a heat-resistant coating have been disclosed by U.S. Pat. No. 4,915,751, Japanese Patent Publication No. 3-1279 (1991), and Japanese Patent Laid-Open No. 2-270904 (1990). U.S. Pat. No. 4,915,751 has disclosed a two-step method of treating a stainless foil at a temperature ranging from 900.degree. C. to 960.degree. C. and at a temperature ranging from 960.degree. C. to 1000.degree. C. to give an alumina whisker. Japanese Patent Publication No. 3-1279 (1991) has disclosed a method of treating a stainless steel foil containing Mg at a temperature ranging from 1000.degree. C. to 1150.degree. C. in vacuum or under a hydrogen atmosphere, and treating the resultant foil under a carbon dioxide atmosphere. Japanese Patent Laid-Open No. 2-270904 (1990) has disclosed a method of treating at a temperature ranging from 950.degree. C. to 1350.degree. C. under an oxidizing atmosphere such as air, oxygen, carbon dioxide, a mixture of hydrogen and water vapor, etc.
However, the method disclosed in U.S. Pat. No. 4,915,751 requires two steps of heat treatments that make temperature control difficult and that also increase an operational cost. The method disclosed in Japanese Patent Publication No. 3-1279 (1991) is applicable only to stainless steel containing magnesium. Moreover, it takes time in the surface treatment process. Both methods disclosed in U.S. Pat. No. 4,915,751 and Japanese Patent Publication No. 3-1279 (1991) are applied to poreless stainless steel manufactured by melting and subsequent rolling.
Though Japanese Patent Laid-Open No. 2-270904 (1990) has disclosed a method of surface treatment under an atmosphere of a mixture of hydrogen and water vapor, specific conditions of the surface treatment have not been disclosed. Moreover, the coating thus obtained does not have satisfactory durability.